In this specification, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date part of common general knowledge, or known to be relevant to an attempt to solve any problem with which this specification is concerned.
Mass spectrometers are specialist devices used to measure or analyse the mass-to-charge ratio of charged particles for the determination of the elemental composition of a sample or molecule containing the charged particles.
A number of different techniques are used for such measurement purposes. One form of mass spectrometry involves the use of an inductively coupled plasma (ICP) torch for generating a plasma field into which a sample to be measured or analysed is introduced. In this form, the plasma vaporises and ionizes the sample so that ions from the sample can be introduced to a mass spectrometer for measurement/analysis.
As the mass spectrometer requires a vacuum in which to operate, the extraction and transfer of ions from the plasma involves a fraction of the ions formed by the plasma passing through an aperture of approximately 1 mm in size provided in a sampler, and then through an aperture of approximately 0.4 mm in size provided in a skimmer (typically referred to as sampler and skimmer cones respectively).
A number of problems are known to exist with prior art mass spectrometer arrangements, which have been observed to reduce their measurement sensitivity.
For example, in the case of plasma mass spectrometry, typical plasma oscillating frequencies are 27 or 40 MHz. Plasma produced by balanced, symmetrically driven, or interlaced coils arrangements is considered to be quasi-neutral, having a relatively low oscillating plasma potential.
However, due to differences in electron mobility as compared with ion mobility, the plasma may in some cases obtain a low positive direct current potential while traveling between the sampler and skimmer cones.
It is thought that this can occur as a result of electrons moving faster than ions when leaving the plasma.
A phenomenon known as ambipolar drift has also been observed to introduce an excessive number of positive ions as compared with the number of electrons during expansion of the plasma jet downstream of the sampler cone.
This can be problematic when charged plasma passes through the skimmer when the skimmer is arranged in a grounded configuration. In such cases, the plasma tends to readjust its potential to a lower state. Accordingly, the plasma has a tendency to eject an excessive amount of ions from the plasma thereby inducing ion recombination with the grounded skimmer. In these situations, ion losses and a drop in measurement sensitivity is almost inevitable.
Another problem with prior art arrangements is collisional scattering. Mass-spectrometers normally operate in a residual gas atmosphere, where gas particles of collisional gases often collide with passing ions which divert or scatter the ions from their intended direction of travel. Collisions of this nature can result in reduced signal sensitivity. Some mass spectrometers utilise specific collisional/reactive cells (a pressurized atmosphere often arranged in conjunction with multi-pole ion guidance systems) to manipulate, control and/or filter the ion beam. In such cases, collisional scatter also becomes a problem where such collisional gases are held under pressure.